Subtracting with minuends to 20 uses strategies including counting back, counting up, and relating to known addition facts. Understanding that subtraction 'undoes' addition is crucial.
Use ten frames, number lines, and physical objects. Model both 'take away' and 'how many more' contexts. Connect explicitly to addition facts.
You already know how to subtract within 10 — taking away small numbers from numbers up to 10. Subtracting within 20 extends that skill, but the key leap isn't just using bigger numbers: it's discovering smarter strategies that make the work much easier.
The most powerful strategy is called think-addition (or "think of the missing addend"). Instead of counting backward from 13 to find 13 − 8, you ask yourself: "8 plus *what* makes 13?" If you know that 8 + 5 = 13, then 13 − 8 = 5. Subtraction and addition are two sides of the same coin — this is what the Core Idea means when it says subtraction "undoes" addition. Every subtraction fact is secretly an addition fact waiting to be used.
A ten frame makes this visual. Think of 13 as a full row of 10 plus 3 more. To subtract 5, you first take the 3 from the bottom row, and then take 2 more from the ten — leaving 8. This "make ten" thinking breaks bigger subtractions into two smaller ones you already know. For 15 − 6: take 5 to get to 10, then take 1 more, leaving 9. You're using the structure of 10 as a stepping stone.
Counting up is another efficient approach. Instead of counting backward from 17 to find 17 − 9, you start at 9 and count up: "10, 11, 12, 13, 14, 15, 16, 17" — that's 8 steps. You arrive at the answer 8 by adding forward rather than subtracting backward. This is especially useful when the two numbers are close together (like 15 − 12), where counting up takes very few steps.
The big picture is this: subtraction isn't just "take away." Sometimes it means "how many more does one number have than another?" and sometimes it means "how far is it between two numbers?" Depending on which way you think about it, a different strategy becomes easiest. Having several strategies — think-addition, make-ten, count up, count back — lets you pick the right tool for each problem.